Available menu items

Hi,

I'm using Allegro CL 6.0 for Windows.  I'd like to dynamically change
the availability of menu items in an application I'm building, but I
can't quite figure out how to do it cleanly.  The way I think it works
is that if the 'available' slot in a menu item is non-nil, the menu
item is available, otherwise not.  Is there a way to have a menu item
test the state of the interface before its menu is displayed, in
event-driven fashion, or do I need to change the 'available' slot
explicitly?  Ideally I'd just put a function in the 'available' slot
to be called at the appropriate time, but that doesn't seem to work.

Thanks,
Rob
-- 
Rob St. Amant
http://www4.ncsu.edu/~stamant